Television



Nov. 23, 1937. J, D. MCGEE 2,100,259

TELEVI S ION Filed May 3, 1934 (kcuuarm HanuLAToR jiy. 3.

IN we Wok.

Patented Nov. 23, 1937 UNIED STATES PATENT OFFICE TELEVISION James Dwyer McGee, Ealing, London, England,

assignor to Electric and Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application May 3, 1934, Serial No. 723,633 In Great Britain May 5, 1933 8 Claims. (01. 178- 6) The present invention relates to television and is particularly concerned with a transmission system involving the use of a cathode ray tube.

It is an object of the present invention to pro- 5 vide an electrical method of transmitting images of objects of very weak light intensity.

According to the present invention there is provided a method of transmitting images of objects wherein an optical image of the object to be transmitted is formed upon a photo-electrically 'The present invention also provides a method of transmitting images of objects wherein an optical image of the object to be transmitted is formed upon a photo-electrically active surface of a screen composed of a multiplicity of relatively insulated elements, the screen is scanned upon the opposite side thereof to the photo-electrically active surface by a primary beam of electrons which serves to bring each element, in turn, tooan equilibrium potential which is substantially the same for all elements and which is stable when the number of electrons reaching the element in the primary beam is equal to the number leaving the element by secondary emission, each element acquiring, in between successive scans, a poten- 35 tial difiering from said equilibrium potential by and shielded photo-electrically active elements,

means for scanning said screen with a primary beam of electrons, an electrode for collecting the electrons emitted photo-electrically from the elements and an electrode for collecting secondary 50 electrons emitted ,.from said screen owing to bombardment by the primary beam.

The screen is maintained at a negative potential relative to the two said electrodes and means are preferably provided whereby the value of the 55 voltage serving to accelerate the primary electron beam can be adjusted for the purpose of adjusting the value of this negative potential;

According to another feature of the present invention there is provided an optical system adapted to project an image of the object to be transmitted upon a photo-electrically active side of a mosaicscreen, means for scanning the opposite side of the screen with a primary beam of electrons, two electrodes arranged adjacent the screen on opposite sides thereof, and a picture signal circuit which includes the electrode which is disposed on the side of the screen opposite to that upon which the optical image is projected.

According to another feature of this invention there is provided television apparatus comprising a cathode ray tube having, within a sealed envelope, a cathode, a mosaic screen of relatively insulated elements, two electrodes, one of which is arranged on the side ofv the screen nearer the cathode and the other of which is arranged on the side of the screen remote from the, cathode and means for making connection from the outside of said envelope with each of said electrodes, wherein each of the elements of the screen has a photo-electrically active surface on the side remote from the cathode and an electrically conductive surface, in electrical connection with the photo-electric surface, upon the side nearer the cathode.

According to another feature of the invention there is provided television transmitting apparatus including a cathode ray tube having a mosaic screen comprising insulated elements each of which is coated on one side with photo-electric material and means for scanning said elements on the sides opposite to those bearing the photoelectric materlal.

The elements are preferably in the form of metal rods (for example of aluminium) insulated from one another and having one of their end surfaces coated with photo-electrically active ma terial.

A constructional embodiment of the invention will now be described by way of example, ref- Referring now to Fig. 1, a television transmitter comprises a cathode ray tube i the envelope of which is in the form of two glass tubes 2 and 3 sealed end to end so that their axes are colinear. The tubes are of roughly equal lengths but of difi'erent diameters. Into the closed end of the smaller tube 3 is sealed a pinch l which carries supports for a cathode 5, a modulating electrode 6, a cylindrical first anode 1 (which takes part in focusing the electron beam emitted from the cathode) and a cylindrical anode 8. The supports for the cathode, the modulating electrode and the auxiliary electrode also serve as leads to these electrodes whilst a lead to the anode 8 is taken through the side wall of the smaller glass tube 3. All of these electrodes are situated axially within the envelope. If desired the anode 8 may be in the form of a metallic deposit on the inner walls of the envelope.

The cathode 5 may be of any suitable kind but preferably is of shallow pill-box shape, electrons being emitted from the circular base which is coated with an electron emitting substance such as a mixture of barium and strontium oxides and which is indirectly heated by means of a coil placed within the box.

The larger glass tube-2 is closed with a transparent plane glass window 9 disposed normally to the axis of the tube. Parallel to and near this glass window 9 is disposed a grid electrode ID in the form of a wire mesh screen substantially filling the larger tube. Close to the grid electrode NJ, on the opposite side thereof from the glass window 9, is disposed an opaque mosaic electrode i! which will be described in greater detail later, and between the mosaic electrode II and the anode 8 is disposed an electrode I2 in the form of a metallic ring of diameter nearly equal to the internal diameter of the larger tube 2.

Leads to the grid and ring electrodes l and I2 respectively are taken through the side walls of the larger tube. but no connection is made to the mosaic electrode II, this electrode being insulated from all other electrodes.

The mosaic electrode ii, a portion of which is shown in greater detail in Fig. 2, is in the form of a disc of diameter nearly equal to the internal diameter of the larger glass tube. The disc is composed of a large number of thin aluminium wires or elements i 3 of length equal to the thickness of the disc, placed side by side and insulated and shielded from each other.

Each of the short aluminium wires is coated with a thin insulating layer H which in turn is coated with a conductive layer IS. The conductive layers of the wires are in contact with one another and thus form a continuous conductor which may be used to shield the elements It elec-' trically from one another. with this arrangement the elements have a small capacity to each other but have each a large capacity to the common conductor.

The mosaic electrode may be made by coating an aluminium wire with aluminium oxide by anodic oxidation. The aluminium oxide may then be coated with a thin conducting film by drawingthe wire through a bath of molten metal or by evaporating a metal such as silver on to the oxidized wire or by painting a film of liquid silver on to the oxidized wire and baking the whole. Alternatively the elements may be of almost any conductive metal coated with an insulating layer which in turn is coated with a conductive layer. After coating the wire in any amazes of the ways described the wire is cut into suitable lengths and packed to form a mosaic screen of about 10 elements per square inch. One end or each element i3 faces towards the grid electrode I0 and the other end towards the cathode 5 of the tube. The ends facing the grid electrode l0 are coated with a photo-electric layer l6 so that this face of the mosaic electrode II is covered with sm all quantities of photo-electric material insulated from one another. The ends of the elements l3 facing the cathode 5 are uncoated.

Means, such as two pairs of deflecting coils (one pair of which is shown at H), are provided for deflecting the cathode ray so as to scan the uncoated face of the mosaic electrode, about 20 times per second, in approximately straight parallel lines. The deflection may be effected either electrostatically or electromagnetically.

The grid electrode l0 and the anode 8 are earthed, the cathode 5 and modulating electrode 6 are maintained at a negative potential, which may for example be 3000 volts, with respect to the anode 8, and the auxiliary electrode 1 may be maintained for example at about 2200 volts negative with respect to the anode 8. The ring electrode I2 is connected to earth through a high resistance l8 and the ends of the resistance It are connected to the input of an amplifier represented in Fig. 1 by a valve I 9.

Signals amplified in the amplifier H are utilized to modulate a carrier wave generated in an oscillator 25, the signals being superimposed upon the carrier wave with the aidof a modulator 26 and the modulated carrier wave being transmitted in known manner.

In order to transmit pictures, an image of the object to be transmitted is optically projected, by means of a lens 20, through the plane glass window 9 and through the grid electrode ill on to that face of the mosaic electrode H which is coated with photo-electric material, whilst the opposite uncoated face of the mosaic electrode H is scanned by the cathode ray 2| in the usual way.

In order to understand the operation of the system let it be assumed, to commence with, that each element l3 of the mosaic electrode l I is at earth potential, that is at the same potential as the anode 8. Then the velocity of the electrons in the scanning, or primary, beam 2| remains substantially constant whilst the electrons pass from the anode to any one element l3 of the mosaic electrode M. This velocity is that due to the anode-cathode potential difference; in the present case 3000 volts.

Now it has been found experimentally that when a primary beam of electrons strikes the uncoated face of an element l3 of the aluminium mosaic electrode H, a secondary emission of electrons occurs from the element owing to bombardment by the primary beam. This secondary emission is represented in Fig. 1 by the arrows 22 and includes electrons scattered or reflected or emitted as a true secondary radiation from the element. The intensity oi the secondary emission, that is to say, the total number of electrons leaving the element per second (irrespective of their velocities) may be either greater or less than the number of electrons reaching the element per second in the primary beam, since it is dependent upon the energy of the primary beam, that is to say the number of primary electronsreaching the they strike the element.

In Fig. 3 there is shown a curve 23 obtained by plotting the ratio of the intensity of the total secondary emission to that of the primary beam as ordinates against the primary beam voltage as abscissae, it being assumed that the number of primary electrons reaching the element per second remains constant at the value indicated by the straight line 24. If the velocity of the primary beam is equivalent to 3000 volts, then less electrons leave the element per second in the secondary emission than reach the element in the primary beam. On the other hand, if the velocity of the primary beam, on striking the element, is equivalent to say 1000 volts, the number of secondary electrons emitted per second is greater than the number of primary electrons reaching the element per second.

In the present examplr the velocity of the electrons in the primary beam 22 (with the screen H at earth potential) is equivalent to 3000 volts. Therefore fewer electrons leave an element l3 than reach it so that the element l3 charges up negatively. Since the element in this manner acquires a. negative potential with respect to the anode 8, the primary beam 22 is s owed down between the anode 8 and the eleme t l3. Th s in turn has the effect of increasing the number of secondary electrons emitted per second from the element l3. A state is thus rapidly reached w en the number of electrons leaving an element I 3 is equal to the number reaching it. In the present case this occurs when the potential of the element l3 has fallen to 750 volts negative with respect to the anode 8, hereinafter referred to as the equilibrium potential, that is, when the velocity has fallen from 3000 to 2250 volts.

As the primary, or scanning, beam 22 sweeps over the elements l3 in succession, each element is brought almost instantaneously to this equilibrium potential whatever potential it may have reached in between successive scans. The equilibrium potential is the same for all elements.

In between successive scans, a photo-electric emission of electrons is occurring from the layers H! on the ends of the elements l3 owing to the optical image focused upon this face of the mosaic electrode II and these electrons are collected by the earthed grid electrode l0.

Provided that the potential of an element l3 does not rise, owing to the photo-electric loss of electrons, above a certain predetermined value, which is negative with respect to the grid potential, the number of electrons emitted photo-electrically by the element will be proportional to the intensity of the light falling upon that element.

Thus in between successive scans each element i3 acquires a potential more positive than the equilibrium potential but not as high as the potential of the grid I owing to the photo-electric emission of electrons, and at each scan the potential of each element is brought back to the fixed equilibrium potential.

The charge acquired by an element during the time that the scanning beam remains on the element is the difference between that due to the primary beam 22 and that due to secondary emission 23 and this must be equal to the charge lost photo-electrically since the last scan.

Now the electrons emitted photo-electrically are collected upon the earthed grid electrode 10, whilst the secondary electrons 23 emitted from the uncoatedends of the elements are collected upon the ring-electrode I2. If a particular element receives no light from the object, then it loses no charge by photo-electric emission between successive scans and at each scan of that element the secondary emission to the ring electrode I 2 remains equal to the primary beam. If an element receives much light from the object then at the subsequent scan of that element the velocity of the primary beam is increased, because the element has acquired a potential more positive than the equilibrium. potential, and the secondary emission is thus reduced until the element recovers its equilibrium potential.

The secondary emission to the ring electrode [2 thus varies, as the scanning beam sweeps over the elements in succession, inversely with the intensity of the light falling upon the elements and by suitable amplifiers this fluctuating current can be converted to a normal type of picture signal suitable for transmission.

Strictly speaking, the picture signal which is generated at each scan'of an element is dependent upon the integral value of the light which has fallen upon the element .since the last scan of the element. In consequence, the picture signals are very much stronger than in the case of certain known systems in which the picture signal is proportional to the instantaneous value of the light falling upon an element.

Scenes of relatively small light intensity may therefore be transmitted by this method.

It will be noted furthermore that the optic axis of the lens 20 is substantially normal to the plane of the mosaic electrode ll so that there is a minimum loss of light owing to reflection at the mosaic electrode.

The mosaic screen II is also disposed normally to the mean direction of the cathode ray so that "keystone distortion of the area scanned by the ray is avoided.

The photo-electric layers l8 on the mosaic electrode H are not damaged by the cathode ray 22 since the photo-electric material is not scanned by the ray.

In order toi'acilitate the capture of secondary electrons the ring electrode l2 may be raised to a positive potential with respect to earth.

The accelerating voltage, that is tosay the potential difierence between the anode and cathode of the tube, may be made variable to permit ready readjustment of the value of the equilibrium potential.

I claim:

1. In a television transmitting system, a mosaic screen having a multiplicity of conductive elements, means for insulating and means for substantially completely electrostatically screening said elements from one another, a coating of photo-electric material upon one side of each of said elements, an optical system for projecting an image of an object to be transmitted upon said material, an electron gun for generating and directing a beam of cathode rays upon the side of said screen opposite to that upon which said image is formed, deflecting means for causing said beam to scan said screen, an electrode separate from said electron gun and from said deflecting means for collecting secondary electrons emitted from said screen, said electrode being located upon the same side of said screen as said electron gun, an output circuit for picture signals associated withsaid electrode and a further electrode located upon the side of said screen upon which said image is formed for receiving photo-electrons emitted from said photo-electric material.

2. For use in television and the like transmission, a mosaic screen formed of a multiplicity of elements each comprising a rod of conducting material, a coating ofphoto-eiectric material :upon one'endoi saidrod a coating of insulating material around the sides of said rod and a conducting layer around said insulating material enveloping substantially the whole length or said rod for substantially completely electrostatically shielding said-rod, said elements being assembled together'with saidconducting layers in electricai contactwith one another.

3. For use in television and the surrounded by an insulating member which is surrounded, in turrnby a conducting screening -member, said elementsbeing assembled together with said screening member in electrical contact with one another and in such a manner that the electrostatic capacity between any two adiacent rods is substantially less than the electrostatic and surrounding substantially the whole length of said rod for substantially completely electrostatically shielding said rod, and a coating of photo-electric material upon one end of said rod,

said elements being assembled together with said conductive coatings in electrical contact with one another. l 1

5. For use in television and the like transmission, a mosaic screen formed-of a multiplicity-0f I elements each comprising a cylindrical rod. of

' metal, a cylindrical-coating of insulating material, in the form ofan oxide of the said metal,

extending around the curved surface of said rod,

a cylindricalcoatingof conductingmaterial extending around said insulating material and surrounding substantially the whole length of said rod for substantially completely electrostatically shielding said rod, and a coating of photo-electric material upon one end of said rod, said elements being assembled together with said conductive coatings in electrical contact with one another.

6. Television and like transmission apparatus comprising an evacuated envelope containing a mosaic screen and an electron gun for directing a beam of electrons upon said screen, said screen being formed of a multiplicity of elements each comprising a rod of conducting material, a coatring :ofphoto-electric material upon one end of said rod, a coating of insulating material around the-sides of said rod and a conducting layer around said insulating material enveloping substantially the whole: lengthof said rod. said elements being assembled together with said con- I I r I r r v ducting. layers in electrical contact, with one anlike transmis sion,a mosaic screen formed of a multiplicity of elements each comprising a conductive member other, said screen being arranged in said envelope with the ends of said rods coated with photo electricmaterial facing away from said electron gun.

a 7. Television and like transmission apparatus comprising an evacuated envelope containing a mosaic screen and an electron gun-'iordirecting I tron gun and a second surface facing away from I said electron gun and being iormed of a multiplicityiof elements each comprising a conductor faces, a coating of photo electrically active material'upon the end faces of said elements which.

lie in said second surface, insulating means. be tween said conductors andscreening means positionedtbetween saidconductors and serving to substantially- .completely: screen said conductors I I electrostatically from one another, the extent and I disposition ofsaid screening means being such that-the electrical capacity between any of said having two opposite end faces :lyingin said suri tially greater than the electrical capacity between anytwo 0f saidconductors- I 8. Television and like transmittingapparatus comprising an evacuated envelope containing a 'mosiac screen and an electron gun for directing I a beam of electrons upon said screen,said screen having a first surface directed towards said electron gun and a'secondsurface facing away from said electron gun and being formed of a multi-- plicity of elements each comprising a conductor over substantially the whole distance between said first and second surfaces, substantially completely electrostatically screening said conductors each from the others.

JAMES DWYER McGEE. 

